The present invention relates to fuel cells; more particularly, to such fuel cells having a solid oxide electrolyte; and most particularly, to such a fuel cell wherein the permeation of oxygen ion to regions of the anode having localized low hydrogen concentration is controlled to prevent localized areas of high oxygen ion concentration which can cause corrosion and failure of the anode.
Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a xe2x80x9csolid oxide fuel cellxe2x80x9d (SOFC). Either pure hydrogen or reformate is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two Oxe2x88x922 ions at the cathode/electrolyte interface. The oxygen ions diffuse through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through the load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived from xe2x80x9creformedxe2x80x9d hydrocarbons, the xe2x80x9creformatexe2x80x9d gas includes CO which is also converted to CO2 at the anode/electrolyte interface.
A single cell is capable of generating a relatively small voltage and wattage, typically about 0.7 volts and less than about 2 watts per cm2 of active area. Therefore, in practice it is usual to stack together in electrical series a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. Adjacent cells are connected electrically by xe2x80x9cinterconnectxe2x80x9d elements in the stack, and the outer surfaces of the anodes and cathodes are electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam or a metallic mesh which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electrical terminals, or xe2x80x9ccurrent collectors,xe2x80x9d connected across a load.
For electrochemical reasons well known to those skilled in the art, an SOFC requires an elevated operating temperature, typically 750xc2x0 C. or greater.
For steric reasons, fuel cells may be rectangular in plan view. Typically, gas flows into and out of the cells through a vertical manifold formed by aligned perforations near the edges of the components, the hydrogen flowing from its inlet manifold to its outlet manifold across the anodes in a first direction, and the oxygen flowing from its inlet manifold to its outlet manifold across the cathodes in a second direction. Thus, fuel cells are typically square in horizontal plan, and the anodes and cathodes have square corners.
A serious problem can arise in operation of a fuel cell formed as just described. The anode typically includes a relatively active metal such as nickel (Ni). In a cell having a square hydrogen flow path, the corners and sides of the square may be stagnant areas in which hydrogen is not readily replenished, allowing the partial pressure of Oxe2x88x922 to build up in the anode and at the anode/electrolyte interface. Oxe2x88x922 which is not scavenged immediately by hydrogen or CO can attack and oxidize nickel in the anode. The mismatch in thermal expansion coefficient between Ni and NiO causes volume changes which can lead to stress and eventual cracking and failure of the cell.
What is needed is a means for preventing the formation of local areas of high oxygen ion concentration at the anode to protect the anode from corrosive attack.
It is a principal object of the present invention to prevent formation of a locally corrosive concentration of Oxe2x88x922 at the anode of a solid oxide fuel cell.
It is a further object of the invention to increase the uniformity of gas distribution over the surface of an anode in a solid oxide fuel cell.
Briefly described, a fuel cell in accordance with the invention has a flow space for the passage of hydrogen gas across the surface of an anode. In the prior art, hydrogen gas may eddy and stagnate in corners or along edges of the flow space, resulting in locally low levels of hydrogen and correspondingly permitting locally high levels of oxygen ion in the anode, which can cause undesirable destructive oxidation of the anode. The invention prevents such destructive oxidation by preventing the buildup of such locally high levels of oxygen ion.
In a first embodiment of the invention, the anode surface itself is shaped in lateral plan to follow the natural contours of gas flow through the space and to eliminate corners or other areas on the anode surface on which gas may eddy and stagnate. Thus, no combustion reaction is possible in these regions of the anode, and oxygen ion therefore is not drawn to these regions.
In a second embodiment, the sidewall of the flow space is shaped by configuring the aperture in the spacer which defines the sidewall of the flow space in such a way that the spacer occludes the otherwise stagnant areas of the rectangular anode, preventing hydrogen from reaching the anode surface in these regions.
In a third embodiment, the anode surface on which gas may eddy and stagnate is dielectric coated in the regions of eddying and stagnation to prevent the migration of hydrogen into the anode.
In a fourth embodiment, the cathode surface corresponding to the anode regions of eddying and stagnation is eliminated to prevent the migration of oxygen ion to the anode in those regions.
In a fifth embodiment, the cathode surface corresponding to the anode regions of eddying and stagnation is much thicker than in cathode regions corresponding to the laminar flow regions of the anode to reduce the migration of oxygen ion to the anode in the stagnation regions.